With recent trends toward miniaturization and weight reduction of electronic equipment, there is also a great demand for a slim and light-weight structure of batteries that are employed as a power source of portable electronic equipment. As a result, lithium-based secondary batteries are now practically available as a battery that is capable of accomplishing high charge/discharge capacity while having a small size.
Lithium ion batteries are being used as power sources for small-size electronic equipment and gradually find a broad range of their applicability extending to electric vehicles (EVs), electric bikes, etc. To this end, there is a need for development of a lithium ion battery having superior high-temperature storage and life span characteristics to those required in conventional small-size batteries. In particular, the lithium ion battery for a power source of hybrid EVs requires improvements of stability and long-term storage characteristics.
Generally, the lithium secondary battery uses a metal oxide such as LiCoO2 as a cathode active material and a carbon material as an anode active material and is fabricated by disposition of a porous polyolefin separator between the anode and the cathode, followed by injection of a non-aqueous electrolyte containing a lithium salt such as LiPF6. Upon charging, lithium ions deintercalate from the cathode active material and intercalate into a carbon layer of the anode. In contrast, upon discharging, lithium ions deintercalate from the carbon layer of the anode and intercalate into the cathode active material. Here, the non-aqueous electrolyte serves as a passage through which lithium ions migrate between the anode and the cathode.
The separator is a crucial element of the lithium ion battery, and it prevents a possible short circuit between electrodes and facilitates ionic migration. In the lithium ion battery, the separator not only serves to electrically isolate the cathode and the anode from each other, but also plays an important role in improvements of battery stability.
The lithium ion battery is a battery that does not adopt metallic lithium and has high stability. However, it disadvantageously requires a variety of measures to prevent battery fire that may result from use of numerous combustible materials as constituent components of the battery, such as Li—LiC, electrolytes, etc. Polyolefin-based separators have been conventionally used as a separator of the lithium ion battery. In order to meet improvement of battery stability, organic/inorganic composite films have been recently introduced as a separator.
Provision of the organic/inorganic composite film using an inorganic material in a coat of the separator increases wettability of the electrolyte and significantly improves electrical and thermal stability of the lithium ion battery.
Unfortunately, the use of the organic/inorganic composite film results in significant deterioration of long-term storage characteristics.
Prevention of resistance increases occurring upon storage of the battery particularly at high temperatures is especially important in the battery as a power source for motor vehicles (EVs and HEVs), requiring instantaneous high-power output. Therefore, there is an urgent need for development of a lithium ion battery with combination of excellent stability and storage characteristics.